1. Field of the Invention
The present invention relates to a method for optically recording information on an information recording medium by means of a laser beam having high energy density and an information recorded medium which is recorded with information by the method.
2. Description of Prior Art
Information recording media for recording and/or reproducing information by the use of a laser beam of high energy density have been developed in recent years and are put into practical use. Such recording media have been widely utilized in various fields, for example, as optical discs such as a video disc and an audio disc as well as memory discs for a large-capacity computer and a large-capacity static image file, a micro-image recording medium, an ultramicroimage recording medium, a microfacsimile, and an optical memory card. Among these recording media, a compact disc (referred to as CD) is now widely used as an audio disc for reproducing a music or the like. The compact disc is used only for reproducing an information represented by a series of pits which were formed previously on a substrate of the disc by a manufacturer of the compact disc when it was manufactured. In more detail, the compact disc is manufactured by molding an appropriate plastic material to give a substrate having a groove thereon (which is utilized for tracking) and a series of pits on the bottom of the groove and then plating a metal thereon to form a reflecting layer. Thus, the compact disc is a recording medium of a read-only type.
Reading of the information from the compact disc is conducted by irradiating the disc under rotation with a laser beam. The information can be reproduced by detecting change of intensity of reflected light which is caused due to presence or absence of pits on the compact disc. The compact disc for reproduction of information only is generally rotated a at fixed linear rate of 1.2 to 1.4 m/sec. according to the predetermined CD standard to read out (reproduce) the recorded information therefrom, and CD is required to allow long time recording (up to 74 min.) using a signal-recording area within the area between inner diameter of 45 mm and outer diameter of 116 mm and using pit width of 0.8 .mu.m and track pitch of 1.6 .mu.m.
As described hereinbefore, audio compact discs are widely used at the present time. Accordingly, a great number of compact disc players for reproducing music or other information recorded on the compact disc are now supplied on market with low prices and widely used. These compact disc players are manufactured to match with the commonly defined requirements of reproducing system.
An information recording medium of DRAW (Direct Read After Write) type on which information can be recorded after the medium is manufactured has been already known. The information recording medium of DRAW type basically comprises a transparent substrate of plastic or glass material and a recording layer provided on the substrate. As materials of the recording layer, metals such as Bi, Sn, In and Te or semimetals, and dyes such as a cyanine dye, a metal complex compound and a quinone dye are generally employed.
Writing of information on the recording medium can be conducted, for example, by irradiating the medium with a laser beam. Under irradiation with the laser beam, the irradiated area of the recording layer of the recording medium absorbs energy of the beam and rise of temperature locally occurs and, as a result, a chemical or physical change such as formation of pits is caused to alter (or change) optical characteristics of the recording layer in the irradiated area, whereby recording of the information is made. Reading of the information from the recording medium is also conducted by irradiating the medium with a laser beam having a less energy. The information can be reproduced by detecting change of reflected or transmitted light corresponding to the change in optical characteristics of the recording layer.
Irradiation of a laser beam on the recording medium for writing of information is carried out on the predermined position of its surface. In order to precisely guide the laser beam to the predetermined irradiation area (generally referred to as "tracking"), the recording medium such as an optical disc is generally provided on its surface with a tracking guide in the form of concave groove (i.e., pregroove) as illustrated in FIG. 1 of an attached drawings. In FIG. 1, "h" indicates a depth of the groove and "w" indicates a half width of the groove (i.e., a width of the groove at a half-depth position of the groove depth).
The previously known recording media of DRAW type are reproduced on a reproducing apparatus which is specifically designed for the employed recording media.
In view of satisfactory availability of unexpensive CD (compact disc) players, it has been recently proposed to employ such CD players for reproduction of information recorded on recording media of DRAW type. For this reason, a number of studies have been made to give a recording medium of DRAW type in which information can be independently recorded after the preparation of the medium and the recorded information can be reproduced utilizing the commercially available CD players. An information recording medium of such type is generally called a compact disc of DRAW type (CD-DRAW).
The compact discs of DRAW type (CD-DRAW) are disclosed in Japanese Patent Provisional Publications No. 61-237239, No. 61-239443 and No. 63-179425. An optical disc described in the Provisional Publication No. 61-237239 comprises a substrate and a recording layer of a dye provided thereon. An optical disc described in the Provisional Publication No. 61-239443 comprises a substrate and a recording layer of an indolenine dye provided thereon. It is described that signals recorded on these optical discs can be reproduced by means of a commercially available CD player. An optical disc described in the Provisional Publication No. 63-179425 comprises a substrate and a recording layer of a composition of a metal and a metal compound and has a high reflectance. It is described that signals recorded thereon can be reproduced by means of a commercially available CD player. In any of these publications, pregrooves provided on the substrates of the compact discs of DRAW type are stated to have such dimension as a half width of 0.8 .mu.m and a depth of 700 angstroms.
In order to enable information recorded on an optical disc to reproduce therefrom by means of a commercially available CD player, it is required for the optical disc to have a high reflectance (preferably not less than 60%) to a laser beam for reproduction. However, a recording layer comprising a metal or a dye which is excellent in recording characteristics such as recording sensitivity generally gas a reflectance in the range of 30 to 40%, while a recording layer comprising different materials which exhibit high reflectances has disadvantage with respect to of recording or reproducing characteristics. To enhance the reflectance of an optical disc using a dye or a metal as a recording material, it is thought that a reflecting layer be provided on the recording layer. From the viewpoint of preparation of a recording layer, use of a dye is advantageous as compared to that of a metal because a recording layer of the dye can be easily formed by coating of the dye solution. Thus, a compact disc of DRAW Type consisting of a substrate, a dye recording layer, a reflecting layer and a protective layer superposed in order, is described in "NIKKEI ELECTRONICS" (No. 465, p. 107, Jan. 23, 1989). In recording the desired information on the compact disc, the disc surface is irradiated with a laser beam from the substrate side to record information by producing rising deformation (protrusion) on a surface of the substrate on the recording layer side. It is stated that the compact disc having the above mentioned structure shows a high reflectance and when signals of CD format are recorded on the disc, the signals can be reproduced utilizing of a commercially available CD player.